1. Field of the Invention
The present invention relates to a process for producing a semiconductor device having a mask programmable ROM (hereinafter referred to as "mask ROM") portion, and more particularly to a process for producing a semiconductor device which provides the step of writing data to the mask ROM portion at the latter stage thereof while enabling the crystalline structure of a semiconductor substrate to be satisfactorily recovered from crystal defect generated therein.
2. Description of the Prior Art
In MOS transistors for use in a mask ROM for data storage, the thresholds of the channel regions below the gate electrodes are all set to the same predetermined value before data writing. Thereafter, a predetermined ion such as boron (B) or phosphorus (P) is implanted selectively into the channel regions of the MOS transistors to change the thresholds thereof in accordance with data to be stored, thereby achieving data writing.
More specifically, by raising the thresholds of the MOS transistors, the MOS transistors which would otherwise be in the ON state can be turned OFF for data writing. Data writing can otherwise be achieved by lowering the thresholds of transistors to change the type of the transistors from the enhancement-type to the depletion-type.
A turnaround time from customer order to shipment of semiconductor devices can be greatly shortened, since the step of the data writing by ion-implantation is provided at the latter stage in the semiconductor device production process. Semiconductor device production processes in which the ion-implantation step is provided after the wiring step are disclosed in Japanese Unexamined Patent Publications No. 55-34443 (1980), No. 59-68964 (1984) and No. 60-28263 (1985).
An example of conventional process for producing a semiconductor device having a mask ROM will hereinafter be described.
As shown in FIG. 20, an LOGOS oxide film 32 for device isolation is formed on a semiconductor substrate 31, followed by the formation of a gate oxide film 33 having a thickness of about 50 angstrom to about 300 angstrom. In turn, a gate electrode 34 is formed on the gate oxide film 33. To form source/drain regions 35 in the semiconductor substrate 31, an impurity ion of the conductivity type opposite to that of the semiconductor substrate 31 is implanted into the semiconductor substrate 31 from the gate electrode side. Thus, a memory transistor is completed. Thereafter, an interlayer insulation film 36 comprising a lower layer of NGS film and an upper layer of BPSG film is formed, and then contact holes are formed as extending through the interlayer insulation film 36 by etching with use of a mask for contact hole formation.
Subsequently, as shown in FIG. 21, a wiring material such as of Al or Al alloy is deposited on the interlayer insulation film 36 including the contact holes, and then patterned into a metal wiring 37 of a desired pattern.
In turn, as shown in FIG. 22, a resist pattern 38 is formed on the semiconductor substrate 31 by using a mask for ROM data writing, and boron ion is implanted into a channel region 39 of the memory-cell transistor. To activate the ion thus implanted, the entire semiconductor substrate is annealed at a temperature of about 450.degree. C. to about 500.degree. C., followed by the formation of a protection film (not shown). Thus, the semiconductor device is completed.
In another conventional production process, a gate electrode 44 and source/drain regions 45 are first formed on a semiconductor substrate 41, and then ROM data is written by ion-implantation with a portion other than the channel region being masked with a resist 46, as shown in FIG. 23.
In the former conventional production processes, however, the annealing process for the activation of the implanted ion is carried out at a temperature of not higher than the melting point (660.degree. C.) of the metal wiring material of Al, for example, at a temperature of about 500.degree. C. This is because the annealing process is performed after the formation of the metal-wiring 37. As a result, crystal defects generated in the semiconductor substrate 31 during the ion-implantation step cannot sufficiently be remedied and, in addition, the ion is not activated enough. In order to attain a desired threshold voltage, it is required to increase the dose of ion-implantation in compensation for the low ion activation. However, this causes junction leakage and the increase of diffusion resistance of the source/drain regions, thereby deteriorating the circuit characteristics.
In still another conventional production process, the activation of the implanted ion is ameliorated by raising the effective annealing temperature with use of such a special annealing method as a laser annealing, electron beam annealing or lamp annealing instead of the aforesaid low temperature annealing (at about 500.degree. C.). However, this process suffers a very low productivity and requires expensive production equipment.
In Japanese Unexamined Patent Publications No. 61-166156 (1986) and No. 4-48778 (1992), there is disclosed a semiconductor device production process in which the ROM data writing is carried out after the formation of the contact hole and before the formation of the metal wiring to solve the aforesaid problems. If the ion-implantation and annealing for ion activation are carried out before the formation of the metal wiring, the annealing temperature can be set higher, thereby eliminating such a problem as the deterioration of the circuit characteristics resulting from the low annealing temperature (below 500.degree. C.). If the high temperature annealing is carried out after the contact hole formation, however, such ions as boron or phosphorus are autodoped to the contact portion on the semiconductor substrate from the interlayer insulation film (e.g., of BPSG). This will lead to an increase in the contact resistance or to a non-linear resistance characteristic, thereby resulting in a contact failure.
Further, where the ROM data writing is carried out after the formation of memory-cell transistor as shown in FIG. 23, the data writing step is followed by a long process sequence. Therefore, such a production process is unsuitable for the fast delivery of semiconductor devices.